[BUG]

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

An API-level issue related to how conversation history is being sent back to the API - it suggests something in the request is modifying thinking blocks that should remain unchanged.
Repeatedly getting this error:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.5.content.14: thinking or redacted_thinking blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CXV3ukrNzAE3i6rUoTmfX"}

What Should Happen?

if a structural error is detected, the CLI should automatically offer to revert to a previous stable state

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.5.content.14: thinking or redacted_thinking blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CXV3ukrNzAE3i6rUoTmfX"}

Steps to Reproduce

I am not certain, as this is happening across multiple prompts and tasks

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.19 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_